The present invention pertains to the manufacture of soda-lime glass. Soda-lime glass has been well known for centuries and, historically, has been made by mixing together soda ash (sodium carbonate) as a source of Na.sub.2 O flux, limestone (calcium carbonate) as a stabilizing oxide, and sand (as the glass-former) along with minor amounts of the other ingredients, such as colorants, fining agents, modifiers, etc.
The use of "wet-batching" in soda-lime glass batches has been used for many years and generally employs attempts to pelletize or granulate the batch feed materials by wetting the mixed ingredients with water or aqueous solutions of caustic. When aqueous caustic is employed as a wet-batching ingredient, it is normally used in relatively small amounts, compared to the soda ash (sodium carbonate) in the batch; the soda ash, provides the major amount of the Na.sub.2 O flux values in the final glass product.
Caustic has been reacted with aluminum oxide source materials for use in glass-making such as is shown in U.S. Pat. No. 3,846,143.
Caustic has been employed with the total glass batch (sand, limestone, minor ingredients) such as in U.S. Pat. No. 3,573,887 and U.S. Pat. No. 3,542,534.
The art believed to be most relevant in regards to reacting caustic with an ingredient in a glass batch formulation, as in the present invention, is believed to be art such as is shown in U.S. Pat. Nos. 3,726,697; 3,573,887; and 3,630,673.
In U.S. Pat. No. 3,726,697 it is taught that an alkali metal hydroxide is reacted with an alkaline earth metal oxide source material in a granulated glass batch material. Also the patent teaches that the flue gases from the glass melting tank may be used in the drying of the glass batch granules.
In U.S. Pat. No. 3,573,887 it is taught that an alkali metal hydroxide is reacted with an alkaline earth metal oxide source material in a glass batch mixture which is formed into a elongated shape and passed directly through a melter to form a continuous strand of molten glass.
In. U.S. Pat. No. 3,630,673 it is taught the reaction of alkali metal hydroxide (e.g. NaOH) with an alkaline earth metal oxide source material (e.g. CaCO.sub.3) gives a product having the formula Na.sub.2 Ca(OH).sub.4 which is useful in a glass batch material.
Other patents illustrative of the state of the art in efforts to prepare improved glass batches, (such as by pelletizing or granulating) are, for example U.S. Pat. Nos. 3,545,988; 3,489,578; 3,001,881; 3,234,034; and 3,081,180.
For a concise, relatively recent treatise on the commercial making of glasses, including soda-lime glasses, it is suggested that the Kirk-Othmer Encylopedia of Chemical Technology, Vol. 10, 1966 be consulted.
It is also known to pre-heat glass batch materials on their journey to the melting pot by using hot flue gas as is shown e.g., in U.S. Pat. Nos. 3,172,648 and 3,459,411. In actual practice it is found to be difficult to avoid particle classification when the glass batch material is passed countercurrently through the flow of hot gas since the gas tends to entrain small particles more than large particles and can separate fine material from the glass batch material.
In the preparation of glass batch material wherein an aqueous alkali metal hydroxide is reacted with an alkaline earth metal oxide source material, it is difficult to assure complete reaction when other materials are present because some of the aqueous alkali metal hydroxide coats the other material and is not in contact with enough of the alkaline earth metal oxide source material to assure complete reaction. On the other hand, if the reaction between the alkali metal hydroxide and the alkaline earth metal oxide source material is carried to completion prior to mixing with the other glass batch materials, then it is difficult to assure that the reaction product is evenly distributed amongst the other particles. The reaction of the aqueous alkali metal hydroxide and the alkaline earth metal oxide source is driven to completion by removing water as the reactants are heated; the reaction is somewhat reversed if water is added back to the reaction product, thereby reforming some alkali metal hydroxide.
It would be a rather tedious and energy-consumming process to completely react the alkali metal hydroxide with the alkaline earth metal oxide source material to obtain a substantially dry reaction product (i.e., having essentially no unreacted alkali metal hydroxide), then mix the reaction product with the remaining glass batch ingredients, adding water to enable pelletizing (which would reverse the reaction by hydrolysis), and then re-heating the pellets at a temperature sufficient to not only dry the pellets but to regain complete reaction of the alkali metal hydroxide and the alkaline earth metal oxide source material.
There is a need for a process by which the reaction of the alkali metal hydroxide and the alkaline earth metal oxide source material can be carried to completion in a substantially uniform manner throughout the total glass batch material while reducing the energy requirements. There is also a need for an improved method of preparing non-decrepitating glass batch pellets of uniform composition. There is also a need for conserving heat energy and for reducing power requirements in the overall process of preparing the pellets and in melting the pellets to form molten glass. There is a further need for abating pollution of the ambient atmosphere normally encountered in glass making operations when particulate matter is carried from the melting operation by gases which flow from the melting operation.
It is an object of this invention to prepare glass batch pellets in which substantially all of the alkali metal oxide values are present as a reaction product of alkali metal hydroxide and alkaline earth metal oxide source material.
It is also an object to prepare the glass batch pellets in such a manner that the reaction product of alkali metal hydroxide and alkaline earth metal oxide source material is substantially uniformly distributed throughout the pellets.
It is another object to utilize the waste heat from a glass batch material during the pellet-forming operation to attain the desired reaction between the alkali metal hydroxide and the alkaline earth metal oxide source material.
Another object is to utilize the waste heat coming from the glass melting operation to pre-heat the batch pellets, thereby salvaging heat energy and reducing the power requirements for maintaining the desired temperature in the melting pot.
It is still another object to utilize the glass batch pellets being fed to the melting operation as scrubbers for entrained fines which are present in the hot gases exiting the glass melting operation.
These and other objects are attained in the practice of the invention described and claimed herein.